Gain, direct current (DC) current, and adjacent channel power (ACP) of an RF power amplifier are functions of the load impedance seen at the output of the power amplifier. Ideally, the load impedance is a constant load; however, in reality, the load impedance may change due to variations in the impedance of the components in the transmit chain following the power amplifier, such as a duplexer, caused by frequency, temperature, and process variations. Load impedance may also vary due to variations in impedance of an antenna of a mobile device caused by proximity of the antenna to foreign metallic objects and a user's body. As a result, RF power amplifier architectures that tend to minimize gain, DC current, and ACP variations due to changes in load impedance are commonly used. Once such architecture is a quadrature RF power amplifier.
The stability of a quadrature RF power amplifier's performance is not as susceptible to output load variations as other RF power amplifier architectures. A quadrature RF power amplifier splits and phase shifts an RF input signal into two quadrature RF signals that are 90 degrees out of phase with each other. The two quadrature RF signals are then amplified, phase shifted back into phase with each other, and then combined to form an RF output signal. The quadrature power amplifier minimizes performance variations by presenting each side of the amplifier with a load that is out of phase with the other side of the amplifier. When one side of the amplifier is presented with a load that causes low gain, the other side is being presented with a load that causes high gain. This is in contrast to a single-ended amplifier in which the amplifier stage is presented with a single load. Hence, the quadrature architecture provides a method of diversifying the load presented to the power amplifier. This diversification results in higher immunity to performance variation resulting from a high voltage standing wave ratio (VSWR) at the power amplifier's output. It is important that the amplitudes of both sides of the power amplifier are closely matched and the 90 degree phase-shift is maintained for proper operation.
Quadrature RF power amplifiers are often constructed into power amplifier modules, which may include a die and a frame. The die may have a common ground connection, such as the backside of the die, which is coupled to elements in the frame, such as a ground flag. This coupling introduces a common inductance through which ground currents from both sides of the power amplifier must flow. This common inductance introduces a feedback path, which produces an imbalance in the amplitude and phase relationship between the two sides of the power amplifier. Thus, there is a need to compensate for the common ground inductance and restore equal amplitude and 90 degree separation to the two sides of the power amplifier.